Lightning protection system for wind turbine blades usually comprises lightning receptor arrangements comprising an external metallic receptor element and an electrically conductive internal block connected to a down conductor of a grounding arrangement of the wind turbine. Once captured lightning currents by the receptor element must be transmitted to the electrically conductive internal block that connect the receptor element to the down conductor.
Lightning receptor arrangements are mainly located at the tip of the blade (such as those disclosed in EP 2 722 522 A1. US 2004/028528 A1 and EP 2 141 356 A1) but they can also be located in other blade regions such as that disclosed in EP 1 664 528 A1.
As shown in FIGS. 1a, 1b and 1c a known lightning receptor arrangement 8 located far from the tip of the wind turbine blade 10 comprises as receptor element a bolt 14 connected to an electrically conductive internal block 15 which is joined to a shell 17 of the wind turbine blade 10 and connected to the down conductor 18.
The bolt 14 and the electrically conductive internal block 15 are mechanically connected by a threaded connection 31 between them and electrically connected both by the threaded connection 31 and by the contact surface 16 (a flat surface) after applying a tightening torque to the bolt 14. In other embodiments the contact surface is a surface of conical geometry.
The contact of the bolt 14 and the electrically conductive internal block 15 through a flat or conical surface is needed for avoiding a physical damage on the threaded connection 31 between them because in this arrangement most of the lightning currents are transferred through the contact surface 16. Otherwise, physical damage on a bolted joint can occur because of the flow of lightning currents.
As shown in FIGS. 3a, 3b and 3c a known lightning receptor arrangement 9 located in a tip region of the wind turbine blade 10 comprises as receptor elements bolts 14 in both sides of the wind turbine blade 10 connected to an electrically conductive internal block 15 which is joined to at least one of the shells 17, 19 and connected to the down conductor 18. The mechanical and electrical connections between bolts 14 and the electrically conductive internal block 15 are similar to those described for the lightning receptor arrangement 8.
The transmission of lightning currents from the receptor elements 14 to the electrically conductive internal block 15 must ensure a correct lightning transmission and permit the adjustment of the receptor elements 14 to the outer surface of the shells 17, 19 to comply with aerodynamic as well as noise reduction requirements.
However in lightning receptor arrangements 8, 9 a gap G between the outer surfaces of receptor elements 14 and shells 17, 19 (see FIGS. 1c and 3c) can take place due to variations of shell thickness as a consequence of tolerances during its manufacturing process causing an incorrect position of the electrically conductive internal block 15 or an unexpected dimensional variation between the head of a receptor element 14 and the surrounding shell.
This invention is directed to the solution of this drawback.